This invention relates to a system for preventing the transmission of a specific first-dialed number in which a specific talking is controlled by a dial number generated by the operation of the dial of a telephone set, and more particularly to a system of controlling the transmission of a specific dial number for positively preventing unauthorized making of a toll call, for example.
In one prior art system for controlling the transmission of a specific number or digit of a dial which is generated by the dialing operation of a telephone set, an additional mechanism is provided for the dial and when a digit "0" of the dial is firstly rotated for the purpose of making a toll call, the telephone is controlled by the additional mechanism. Such system is disclosed, for example, in U.S. Pat. No. 3,761,639. In this system, however, since the control operation of the transmission of all dial numbers is made on the side of the telephone set, there are the following problems. More particularly, in a usual telephone system when a subscriber off-hooks the handset of his telephone set for the purpose of producing an origination, after confirming the off-hooking of the handset, the telephone exchange transmits a dial pulse transmission OK signal or dial tone to the originating telephone set. After receiving the dial tone the subscriber manipulates his dial for transmitting a specific dial number. In this case, there is a time difference of about 500 ms between the time at which a dial tone signal is sent to the telephone set, and the time at which reception the dial number sent from the subscriber is registered in the transmission register in the telephone exchange after the origination has been confirmed thereby. In correct dialing, the originator should operate his dial after receiving the dial tone signal; but when the originator operates his dial without receiving the signal, the number of dial digits received by the telephone exchange and the number of the dial digits produced by the originator will not coincide with each other, thus resulting in incorrect dialing. If such condition were improperly used, the originator could transmit a specific number which is forbidden. Assume now that the numeral "0" is the specific number which is forbidden and that the other numerals can be transmitted by the first dialing operation. Considering the improper transmission of the forbidden digit, if the number "1", for example, is dialed during an interval between the off-hooking of the handset and the arrival of the dial tone signal from the telephone exchange, transmission of all other dial digits would no longer be prevented since the number "1" is the first dialed digit. Under these conditions, the telephone exchange would not receive the number "1". Then, if the number "0" is dialed for the purpose of making a toll call, the dial mechanism would not operate to prevent the forbidden number from being dialed since it is the second rotation of the dial. Accordingly, the telephone exchange transmits the digits starting from "0". Thus, such a mechanism does not serve to prevent the transmission of the number "0".
As described above, it is extremely difficult to prevent improper operation where the transmission of a specific dial number is precluded and an effective solution of this problem has been desired for a long time, yet no effective solution has been made.